1. Field of the Invention
The present invention relates to an optical amplifier for wavelength-multiplexing transmission which amplifies signals of a plurality of wavelengths in a 1.55-xcexcm wavelength band as a single unit in a wavelength multiplexing transmission system.
2. Related Background Art
Conventionally, wavelength-multiplexing transmission is performed using signals of a plurality of wavelengths in a 1.55-xcexcm wavelength band. In such a wavelength-multiplexing transmission system, an optical amplifier for wavelength-multiplexing transmission which amplifies signals of the plurality of wavelengths as a single unit is used. This optical amplifier for wavelength-multiplexing transmission is required to have a high gain, a wide wavelength band capable of optical amplification, flat wavelength dependence of the gain in the wavelength band, a high S/N ratio, a satisfactory noise factor, and a wide dynamic range for input signal level, and has been researched and developed from these viewpoints.
For example, a device disclosed in Japanese Patent Laid-Open No. 5-48207 uses a combination of an optical amplifier whose gain for signals is controlled to a predetermined value, and an optical attenuator provided on the output side of the optical amplifier and having a variable attenuation factor for signals. When the input signal level varies, the output signal level from the optical amplifier also varies. However, a predetermined output signal level from the optical attenuator is maintained by controlling the attenuation factor of the optical attenuator. This widens the dynamic range of input signal level with satisfactory noise factor.
The wavelength dependence of gain with respect to signals in the optical amplifier also depends on the gain. Generally, while controlling the gain of the optical amplifier to obtain a predetermined average gain, a gain equalizer having a fixed gain equalizing characteristic is used to flatten the wavelength dependence of total gain of the optical amplifier and gain equalizer.
For example, a device described in Yoshikazu Saeki et al., xe2x80x9cOptical Fiber Amplifier Incorporating Dynamic Equalizing Functionxe2x80x9d, NEC Technical Journal, Vol. 51, No. 4, pp. 45-48 (1998) combines an input-side optical amplifier, gain equalizer, and output-side optical amplifier in the order named. This gain equalizer has an optical circulator and AWG (Arrayed Waveguide Grating). With this arrangement, the signal level of each wavelength is detected, and the gain equalizing characteristic of the gain equalizer is dynamically adjusted on the basis of the detection result, thereby flattening the wavelength dependence of total gain.
However, a conventional optical amplifier for wavelength-multiplexing transmission has the following problems. In the device disclosed in Japanese Patent Laid-Open No. 5-48207, to maintain constant gain control in the optical amplifier even in a region where the input signal level is high, the optical amplifier need to have high output power, as in a region with low input signal level. In addition, to obtain a predetermined output signal level for each wavelength from the output-side optical amplifier, the attenuation factor of the optical attenuator must be as large as the increase amount in input signal level. Hence, to widen the dynamic range of input signal level, pumping light with higher power must be supplied to the optical amplifier, resulting in an increase in power consumption and a decrease in service life of the pumping light source.
In the device described in the above reference, since the insertion loss of the gain equalizer for flattening the wavelength dependence of total gain is as large as 15 dB, the gain of the optical amplifier must be increased to 30 dB. Additionally, in order to supply high-power pumping light to the input-side optical amplifier, the input-side optical amplifier comprises two pumping light sources. Hence, the device described in this reference also increases power consumption and shortens the service life of the pumping light sources.
The present invention has been made to solve the above problems, and has as its object to provide an optical amplifier for wavelength-multiplexing transmission capable of widening the dynamic range of input signal level without requiring supply of high-power pumping light.
An optical amplifier for wavelength-multiplexing transmission according to the present invention comprises an input-side optical amplifier and an output-side optical amplifier. The input-side optical amplifier has (1) a first amplification optical fiber for amplifying input signals and outputting the amplified signals, (2) first pumping means for supplying first pumping light having power to the first amplification optical fiber, and (3) first pumping light control means for controlling the first pumping means so that an amplification gain of each signal in the first amplification optical fiber decreases in proportion to an increase amount of level of each input signal. The first pumping means supplies, to the first amplification optical fiber, first pumping light having power not more than minimum pumping light power at which the gain of the first amplification optical fiber which has received a minimum input level for the largest number of wavelengths in use causes gain saturation for an increase in input signal level beyond the minimum level. The output-side optical amplifier has (1) a second amplification optical fiber for amplifying the signals input from the input-side optical amplifier and outputting the signals, (2) second pumping means for supplying second pumping light to the second amplification optical fiber, and (3) second pumping light control means for controlling the second pumping means so that the level of each signal output from the second amplification optical fiber has a second gain or target output value.
The optical amplifier for wavelength-multiplexing transmission according to the present invention functions as follows. The input signals are sequentially amplified by the input-side optical amplifier and output-side optical amplifier and output. In optical amplification for signals by the input-side optical amplifier, the first pumping means for supplying the first pumping light to the first amplification optical fiber is controlled by the first pumping light control means such that the amplification gain of each signal in the first amplification optical fiber decreases in proportion to an increase amount of the level of each input signal. The first pumping light control means supplies pumping light having power equal to or smaller than (preferably 90% or less and, more preferably, 80% or less) minimum pumping light power at which the gain of the first amplification optical fiber which has received a minimum input level for the largest number of wavelengths in use causes gain saturation for an increase in input signal level beyond the minimum level. In optical amplification for signals by the output-side optical amplifier, the second pumping means for supplying the second pumping light to the second amplification optical fiber is controlled by the second pumping light control means such that the level of each signal output from the second amplification optical fiber has the second gain or target output value. With such control, without continuously supplying excessively high-power pumping light to maintain the gain of the input-side optical amplifier almost constant across the entire range of input signal level to be used, gain or constant output control can be performed as a whole while minimizing degradation in noise factor and gain flatness.
In the optical amplifier for wavelength-multiplexing transmission according to the present invention, the first pumping light control means controls the first pumping means such that the level of each signal output from the first amplification optical fiber has the first target output value. In this case, the input-side optical amplifier also performs constant output control. Each of the first and second target output values is not limited to one value but has a certain range, so control is performed such that the level of each signal output from each of the input-side optical amplifier and output-side optical amplifier falls within this range. When both the input-side optical amplifier and output-side optical amplifier perform constant output control, the first pumping means is controlled such that the gain of the first amplification optical fiber for the input signal level to the input-side optical amplifier decreases by simple control. With such control, without continuously supplying excessively high-power pumping light to maintain the gain of the input-side optical amplifier almost constant across the entire range of input signal level to be used, gain or constant output control can be performed as a whole while minimizing degradation in noise factor and gain flatness.
The optical amplifier for wavelength-multiplexing transmission according to the present invention further comprises input signal level detection means for detecting the level of each signal input to the input-side optical amplifier, and the first (or second) pumping light control means sets the first (or second) gain or target output value in accordance with the level of each signal detected by the input signal detection means. In this case, in the input-side (or output-side) optical amplifier, the first (or second) gain or target output value in the first (or second) pumping light control means is set on the basis of the input signal level detected by the input signal level detection means, and gain or constant output control is performed such that the output signal level has the set first (or second) gain or target output value. Thus, the wavelength dependence of total gain can be maintained smaller even when the input signal level varies.
The optical amplifier for wavelength-multiplexing transmission according to the present invention further comprises input signal level detection means for detecting the level of each signal input to the input-side optical amplifier, and the first (or second) pumping light control means sets the gain of optical amplification in the first (or second) amplification optical fiber in accordance with the level of each signal detected by the input signal level detection means and controls the first (or second) pumping means on the basis of the gain. In this case, in the input-side (or output-side) optical amplifier, the gain of optical amplification in the first (or second) amplification optical fiber is set on the basis of the input signal level detected by the input signal level detection means, and the first (or second) pumping means is controlled on the basis of the gain. Thus, the input-side (or output-side) optical amplifier performs gain or constant output control such that the output signal level has the first (or second) gain or target output value, so the wavelength dependence of total gain can be maintained small when the input signal level varies.
In this case, preferably, the input-side (output-side) optical amplifier includes a first optical amplification section and second optical amplification section, and the first (or second) pumping light control means sets the gain of optical amplification for each of the first and second optical amplification sections in accordance with the level of each signal detected by the input signal level detection means. In this case, even when complete gain or constant output control cannot be performed by the stand-alone first or second optical amplification section, it can be performed by the input-side (or output-side) optical amplifier as a whole including the firsthand second optical amplification sections.
The optical amplifier for wavelength-multiplexing transmission according to the present invention further comprises input signal level detection means for detecting the level of each signal input to the input-side optical amplifier, an optical attenuator inserted between the input-side optical amplifier and the output-side optical amplifier and having a variable attenuation factor for the signals, and attenuation factor control means for controlling the attenuation factor of the optical attenuator in accordance with the level of each signal detected by the input signal level detection means. In this case, the signals are amplified by the input-side optical amplifier under constant output control, attenuated by the optical attenuator by the attenuation factor corresponding to the input signal level detected by the input signal level detection means, and further amplified by the output-side optical amplifier under constant output control. With this arrangement, the wavelength dependence of total gain can be maintained small even when the input signal level varies.
The optical amplifier for wavelength-multiplexing transmission according to the present invention further comprises a gain equalizer inserted between the input-side optical amplifier and the output-side optical amplifier to equalize a wavelength dependence of gain for signals as a whole. In this case, the signal are amplified by the input-side optical amplifier under constant output control, gain-equalized by the gain equalizer, and further amplified by the output-side optical amplifier under constant output control. With this arrangement, the wavelength dependence of total gain can be further reduced.
In optical amplifier for wavelength-multiplexing transmission according to the present invention, the first amplification optical fiber comprises an Er-doped optical fiber co-doped with Al, and the second amplification optical fiber comprises an Er-doped optical fiber co-doped with Al and P and an Er-doped optical fiber co-doped with Al, which are connected in the order named. In this case, even when the gain of the input-side optical amplifier is small, and the level of each signal input to the output-side optical amplifier is low, the wavelength dependence of total gain can be maintained small. For the second amplification optical fiber, the Er-doped optical fiber co-doped with Al and P and Er-doped optical fiber co-doped with Al may be directly connected, or another amplification optical fiber may be inserted therebetween. The Er-doped optical fiber co-doped with Al is substantially undoped with P.
In the optical amplifier for wavelength-multiplexing transmission according to the present invention, the first pumping means supplies the first pumping light at least from a front side to the first amplification optical fiber. In this case, the inverted distribution state near the signal incident side of the first amplification optical fiber can be made high even when the gain of the input-side optical amplifier is small, and the first amplification optical fiber has a low inverted distribution state as a whole. For this reason, degradation in noise factor can be reduced.